1. Field of the Invention
The present invention relates to a semiconductor light-receiving device.
2. Related Background Art
Conventionally known as a semiconductor light-receiving device are p-i-n type photodiodes. A light-receiving device of this type comprises an n-type semiconductor substrate, an i-type InP buffer layer formed on this substrate, an i-type InP layer, and an InGaAs light-receiving layer disposed between a p-type InP layer and the i-type InP layer. The i-type InP layer is doped with a p-type dopant. Light incident on this semiconductor light-receiving device is absorbed in the light-receiving layer to be converted into current.
The inventor conducted measurements concerning response characteristics of a p-i-n type photodiode. These measurements were carried out in order to obtain response characteristics for a bit rate of about 1.25 Gb/s at various voltages applied across the semiconductor light-receiving device. In the measurements of response characteristics, the intensity of light introduced to the semiconductor light-receiving device was varied among some values.
The inventor found the following problem in the measurements. When the response characteristics are compared with each other over various conditions of applied voltages and light intensity, wave tails occur under conditions of relatively greater light intensity and lower applied voltages. These wave tails tend to disappear when the applied voltage is raised or when the light intensity is lowered. The inventor also found that the wave tails appeared only at falling edges of optical pulses in carefully observing the response characteristics.
Therefore, it is an object of the present invention to provide a semiconductor light-receiving device capable of reducing the occurrence of the wave tails.
The inventor conducted studies concerning the foregoing characteristics. In the measurements of the semiconductor light-receiving device, a reverse bias is applied to the device. The wave tails occur in the lower applied voltages. In view of this fact, it is presumed to relate to the drift of carriers generated in the device. The wave tails also occur in the greater light intensity. In view of this fact, the phenomenon becomes remarkable when the number of generated carriers is large. In addition, the wave tails only occurs in falling edges of optical pulses.
In view of these facts, it can be understood as follows: the wave tails may be caused by a number of carrier pairs, generated by a relatively high light intensity, that cannot move at a sufficient speed due to a relatively low drift voltage.
The semiconductor light-receiving device in accordance with the present invention comprises: (a) a first InP layer; (b) a second InP layer; and (c) an InGaAs layer light-receiving layer. The first InP layer includes a semiconductor region having a first conductive type. The first InP layer is provided on the second InP layer and the second InP layer includes a semiconductor regions having a second conductive type different from the first conductive type. The InGaAs light-receiving layer is provided between the first and second InP layers. The semiconductor region having a first conductive type in the second InP layer has a carrier concentration of 1xc3x971017 cmxe2x88x923 or higher.
Since the first conductive type semiconductor region has a carrier concentration of 1xc3x971017 cmxe2x88x923 or higher, the depletion region, generated by the applied reverse bias, is sufficiently reduced in the first InP layer. As a consequence, this depletion layer is mainly generated within the light-receiving layer in which electron-hole pairs are generated. Hence, the applied voltage can be fully utilized for drifting the generated carriers.
Thus configured first InP layer is provided on the substrate. The InGaAs light-receiving layer may have an i-type semiconductor region.
The semiconductor light-receiving device may be configured such that the first InP layer has a p-type semiconductor region, the second InP layer has an n-type semiconductor region, the InGaAs light-receiving layer has an i-type semiconductor region, and the substrate is an n-type semiconductor substrate.
The InGaAs light-receiving layer may have a profile of a first conductive type dopant concentration decreasing in a direction from the second InP layer to the first InP layer. This dopant profile defines the width of a high resistance region and the inclination of the impurity distribution curve in the light-receiving layer.
The substrate may be an InP substrate having a pair of surfaces. On one surface of the substrate, the semiconductor layers above are provided. The semiconductor light-receiving device may be a back entrance type device. The other surface may have a lens portion in a first region. The lens portion can provide the InGaAs light-receiving layer with condensed light. The lens portion is separated from the light receiving layer by the second conductive type InP substrate. The thickness of the second conductive type InP substrate can be, therefore, related to the focal length of the lens portion. The lens portion may include a monolithic lens.
The projection of the lens portion is made onto the other surface of the substrate to define a first projection area thereon. The projection of the second conductive type region is made onto one surface of the substrate to define a second projection area thereon. The first projection area may be larger than the second projection area. The smaller second projection area reduces the parasitic capacitance occurring due to the second conduction type semiconductor region.
The above-mentioned object and other objects, features, and advantages of the present invention will be clarified more easily from the following detailed descriptions of preferred embodiments of the present invention set forth with reference to the accompanying drawings.